Problem rotating object around particular axis

I am trying to write a script which will rotate my 'grabber' object around its y-axis by 180 degrees when it collides with another object. The other objects are smaller objects moving along a conveyor belt. The grabber should grab onto an item, rotate around its y axis 180 degrees, and release the object.

My script successfully rotates by 180 degrees on the y axis every time it collides with a conveyor object, but at the same time changes the x and z axis rotations, which I do not want. The resulting rotation is this -

As you can see, the grabber is no longer tilted. How would I get the object to rotate ONLY on its y axis? So that no matter how it is oriented, it would always rotate on its local y axis by 180 degrees when it collides?

The most relevant part of the code below is -

  //Specify the desired rotation
              deltaRot = Quaternion.AngleAxis(transform.rotation.eulerAngles.y + 180f, transform.up);

My script-

 using UnityEngine;
 using System.Collections;
 
 public class MagnetScript : MonoBehaviour {
 
     //The grab positions
     private Transform[] grabPositions = new Transform[2];
     private Transform grabPosition; 
 
     //The obj to grab and it's rb
     private GameObject grabObj;
     private Rigidbody rb;
 
     //Our Finite State machine
     private enum State {idle, grabbing, rotating, releasing};
     private State state = State.idle;
 
     //Our grabber turn speed
     public float turnSpeed;
 
     //Orignal grabber rotation and desired rotation
     private Quaternion originalRotation, desiredRotation, deltaRot;
 
     //The layer mask to define
     int layerMask;
 
     float buffer = 0.01f;
 
     void Start()
     {
         //Define the two grab positions
         grabPositions[0] = transform.Find("GrabPosition1").transform;
         grabPositions[1] = transform.Find("GrabPosition2").transform;
 
         //Define the layer mask (conveyor grabObjects)
         layerMask = LayerMask.NameToLayer("ConveyorObjects");
     }
     void FixedUpdate()
     {
         if (state == State.grabbing) {
 
             if (Vector3.Distance (grabObj.transform.position, grabPositions [0].position) > Vector3.Distance (grabObj.transform.position, grabPositions [1].position)) {
                 grabPosition = grabPositions [1];
             } else
                 grabPosition = grabPositions [0];
 
             //Hold the grabObject in the grab position
             rb.MovePosition (grabPosition.position);
 
             //Specify the desired rotation
             deltaRot = Quaternion.AngleAxis(transform.rotation.eulerAngles.y + 180f, transform.up);
 
             //Set State to rotating
             state = State.rotating;
         }
 
         if(state == State.rotating)
         {
 
             //Hold the grabObject in the grab position
             rb.MovePosition (grabPosition.position);
 
             //Slerp the rotation
             transform.rotation = Quaternion.Slerp( transform.rotation, deltaRot , Time.deltaTime * turnSpeed);
 
             //Calculate the relative position to the og position
             Quaternion relative = Quaternion.Inverse(deltaRot) * transform.rotation;
 
             //if the relativePos is less than the threshold, then release the grabObject
             if ( relative.eulerAngles.y < buffer ) {
                 state = State.releasing;
             }
         }
 
         //Release the grabObject, turn gravity back on
         if (state == State.releasing){
             rb.useGravity = true;
             rb.isKinematic = false;
             state = State.idle;
             grabObj = null;
             grabPosition = null;
         }
     }
 
     void OnTriggerEnter(Collider col)
     {
         //Grab grabObject if we are not already grabbing something
         if (state == State.idle) {
 
             //Only grab the conveyor grabObjects
             if (col.gameObject.layer == layerMask) {
 
                 state = State.grabbing;
 
                 //Get a reference to the grabObject being grabbed
                 grabObj = col.gameObject;
 
                 //Get grabObjects rigidbody and turn off use gravity while we grab
                 rb = col.gameObject.GetComponent<Rigidbody> ();
                 rb.useGravity = false;
                 rb.isKinematic = true;
             }
         }
     }
         
 }